Abstract. The sensitivity of the process parameters of the Biosphere Energy Transfer HYdrology (BETHY) model to choices of atmospheric
concentration network, high frequency terrestrial fluxes, and the choice of
flux measurement network is investigated by using a carbon cycle data
assimilation system. We use BETHY-generated fluxes as a proxy of flux
measurements. Results show that monthly mean or low-frequency observations
of CO2 concentration provide strong constraints on parameters relevant
for net flux (NEP) but only weak constraints for parameters controlling
gross fluxes. The use of high-frequency CO2 concentration observations,
which has led to great refinement of spatial scales in inversions of net
flux, adds little to the observing system in the Carbon Cycle Data Assimilation System (CCDAS) case. This unexpected
result is explained by the fact that the stations of the CO2
concentration network we use are not well placed to measure such high
frequency signals. Indeed, CO2 concentration sensitivities relevant for
such high frequency fluxes are found to be largely confined in the vicinity
of the corresponding fluxes, and are therefore not well observed by
background monitoring stations. In contrast, our results clearly show the
potential of flux measurements to better constrain the model parameters
relevant for gross primary productivity (GPP) and net primary productivity
(NPP). Given uncertainties in the spatial description of ecosystem
functions, we recommend a combined observing strategy.